Industrial sensors and control--The basics--Part IV

The operating principle of a displacement sensor using Y-guide probes is illustrated in Fig. 6.15. The most common Y-guide probe is a bifurcated fiber bundle. The light emitted from one bundle is back reflected by the object to be measured and collected by another bundle (receiving fibers). As a result, the returned light at the detector is intensity-modulated to a degree dependent on the distance between the end of the fiber bundle and the object. The sensitivity and the dynamic range are determined by the geometrical arrangement of the array of fiber bundles and by both the number and type of the fibers. Figure 6.16 shows the relative intensity of the returned light as a function of distance for three typical arrangements: random, hemispherical, and concentric circle arrays. The intensities increase with distance and reach a peak at a certain discrete distance. After that, the intensities fall off very slowly. Most sensors use the high-sensitivity regions in these curves. Among the three arrangements, the random array has the highest sensitivity but the narrowest dynamic range. The displacement sensor using the Y-guide probe provides a resolution of 0.1?m, linearity within 5 percent, and a dynamic range of 100?m displacement. Y-guide probe displacement sensors are well-suited for robotics applications as position sensors and for gauging and surface assessment since they have high sensitivity to small distances.

One profound problem of this type of displacement sensor is the measuring error arising from the variation in parasitic losses along the optical transmission line. Recalibration is required if the optical path is interrupted, which limits the range of possible applications. In order to overcome this problem, a line-loss-independent displacement sensor with an electrical subcarrier phase encoder has been implemented. In this sensor, the light from an LED modulated at 160 MHz is coupled into the fiber bundle and divided into two optical paths. One of the paths is provided with a fixed retro-reflector at its end. The light through the other is reflected by the object. The two beams are returned to the two photodiodes separately. Each signal, converted into an electric voltage, is electrically heterodyned into an intermediate frequency at 455 kHz. Then, the two signals are fed to a digital phase comparator, the output of which is proportional to the path distance. The resolution of the optical path difference is about 0.3 mm, but improvement of the receiver electronics will provide a higher resolution.

PROCESS CONTROL SENSORS MEASURING AND MONITORING LIQUID FLOW

According to the laws of fluid mechanics, an obstruction inserted in a flow stream creates a periodic turbulence behind it. The frequency of shedding the turbulent vortices is directly proportional to the flow velocity. The flow sensor in Fig. 6.17 has a sensing element consisting of a thin metallic obstruction and a downstream metallic bar attached to a multimode fiber-microbend sensor. As illustrated in Fig. 6.18, the vortex pressure produced at the metallic bar is transferred, through a diaphragm at the pipe wall that serves as both a seal and a pivot for the bar, to the microbend sensor located outside the process line pipe. The microbend sensor converts the time-varying mechanical force caused by the vortex shedding into a corresponding intensity modulation of the light. Therefore, the frequency of the signal converted into the electric volt- age at the detector provides the flow-velocity information. This flow sensor has the advantage that the measuring accuracy is essentially independent of any changes in the fluid temperature, viscosity, or density, and in the light source intensity. According to the specifications for typical optical vortex-shedding flow sensors, flow rate can be measured over a Reynolds number range from 5 µ 103 to 6000 µ 103 at temperatures from µ100 to +600°C. This range is high compared to that of conventional flow meters. In addition, an accuracy of ±0.4 and ±0.7 percent, respectively, is obtained for liquids and gases with Reynolds numbers above 10,000.

Flow Sensor Detecting Small Air Bubbles for Process Control in Manufacturing

Another optical-fiber flow sensor employed in manufacturing process control monitors a two- fluid mixture (Fig. 6.19). The sensor can distinguish between moving bubbles and liquid in the flow stream and display the void fraction, namely, the ratio of gas volume to the total volume.

The principle of operation is quite simple. The light from the LED is guided by the optical fiber to the sensing element, in which the end portion of the fiber is mounted in a stainless steel needle of 2.8-mm outer diameter. When liquid is in contact with the end of the fiber, light enters the fluid efficiently and very little light is returned. However, when a gas bubble is present, a significant fraction of light is reflected back. With this technique, bubbles as small as 50 µm may be detected with an accuracy of better than 5 percent and a response time of only 10 µs.

Potential applications of this flow sensor for the control of processes in manufacturing systems are widespread—for example, detection of gas plugs in production wells in the oil industry and detection of fermenters and distillers in the blood-processing and pharmaceutical industries.

An optical-fiber flow sensor for a two-phase mixture based on Y-guide probes is shown in Fig. 6.20. Two Y-guide probes are placed at different points along the flow stream to emit the input light and to pick up the retro-reflected light from moving solid particles in the flow. The delay time between the signals of the two probes is determined by the average velocity of the moving particles. Therefore, measurement of the delay time by a conventional correlation technique provides the flow velocity. An accuracy of better than ±1 percent and a dynamic range of 20:1 are obtained for flow velocities up to 10 m/s. A potential problem of such flow sensors for two-phase mixtures is poor long-term stability, because the optical fibers are inserted into the process fluid pipes.

Liquid Level Sensors in Manufacturing Process Control for Petroleum and Chemical Plants

Several optical-fiber liquid level sensors developed in recent years have been based on direct interaction between the light and liquid. The most common method in commercial products employs a prism attached to the ends of two single optical fibers (Fig. 6.21). The input light from an LED is totally internally reflected and returns to the output fiber when the prism is in air. However, when the prism is immersed in liquid, the light refracts into the fluid with low reflection, resulting in negligible returned light. Thus, this device works as a liquid level switch. The sensitivity of the sensor is determined by the contrast ratio, which depends on the refractive index of the liquid. Typical examples of signal output change for liquids with different refractive indices are indicated in Table 6.2.

The output loss stays at a constant value of 33 dB for refractive indices higher than 1.40. The signal output of a well-designed sensor can be switched for a change in liquid level of only 0.1 mm.

Problems to be solved for this sensor are dirt contamination on the prism surface and bubbles in the liquid. Enclosing the sensing element with a fine filter helps keep it clean and simultaneously reduces level fluctuations caused by bubbles. Since optical-fiber liquid level sensors have the advantages of low cost and electrical isolation, their use is widespread in petroleum and chemical plants, where the hazardous environment causes difficulties with conventional sensors. They are used, for example, to monitor storage tanks in a petroleum plant.

Another optical-fiber liquid level sensor, developed for the measurement of boiler-drum water level, employs a triangularly shaped gauge through which red and green light beams pass. The beams are deflected as it fills with water, so that the green light passes through an aperture. In the absence of water, only red light passes through. Optical fibers transmit red or green light from individual gauges to a plant control room located up to 150 m from the boiler drum (Fig. 6.22). The water level in the drum is displayed digitally.

This liquid level sensor operates at temperatures up to 170°C and pressures up to 3200 lb/in2 gauge. Many sensor units are installed in the boiler drum, and most have been operating for seven years. This sensor is maintenance-free, fail-safe, and highly reliable.

On-line Measuring and Monitoring of Gas by Spectroscopy

An optical spectrometer or optical filtering unit is often required for chemical sensors because the spectral characteristics of absorbed, fluorescent, or reflected light indicate the presence, absence, or precise concentration of a particular chemical species (Fig. 6.23).

Sensing of chemical parameters via fibers is usually done by monitoring changes in a suitably selected optical property--absorbance, reflectance, scattering (turbidity), or luminescence (fluorescence or phosphorescence), depending on the particular device. Changes in parameters such as the refractive index may also be employed for sensing purposes. The change in light intensity due to absorption is determined by the number of absorbing species in the optical path, and is related to the concentration C of the absorbing species by the Beer-Lambert relationship. This law describes an exponential reduction of light intensity with distance (and also concentration) along the optical path. Expressed logarithmically,

where A is the optical absorbance, l is the path length of the light, η is the molar absorptivity, and I0 and I are the incident and transmitted light, respectively. For absorption measurements via optical fibers, the medium normally must be optically transparent.

An accurate method for the detection of leakage of flammable gases such as methane (CH4), propane (C3H8), and ethylene (C2H4) is vital in gas and petrochemical plants in order to avoid serious accidents. The recent introduction of low-loss fiber into spectroscopic measurements of these gases offers many advantages for process control in manufacturing:

• Long-distance remote sensing

• On-line measurement and monitoring

• Low cost

• High reliability

The most commonly used method at present is to carry the sample back to the measuring laboratory for analysis. Alternatively, numerous spectrometers may be used at various points around the factory. The new advances in spectroscopic measurements allow even CH4 to be observed at a distance of 10 km with a detection sensitivity as low as 5 percent of the lower explosion limit (LEL) concentration. The optical-fiber gas measuring sys- tem employs an absorption spectroscopy technique, with the light passing through a gas- detection cell for analysis. The overtone absorption bands of a number of flammable gases are located in the near-infrared range (Fig. 6.24).

The optical gas sensing system can deal with a maximum of 30 detection cells (Fig. 6.25). The species to be measured are CH4, C3H8, and C2H4 molecules. Light from a halogen lamp (an infrared light source) is distributed into a bundle of 30 single optical fibers. Each of the distributed beams is transmitted through a 1-km length of fiber to a corresponding gas detection cell. The receiving unit is constructed of three optical switches, a rotating sector with four optical interference filters, and three Ge photodiodes. Each optical switch can select any ten returned beams by specifying the number of the cell. The peak transmission wavelength of the optical filter incorporated in the sensor is 1.666 ?m for CH4, 1.690 ?m for C3H8, 1.625 ?m for C2H2, and 1.600 ?m for a reference beam. After conversion to electrical signals, the signal amplitudes for the three gases are normalized by the refer- ence amplitude. Then the concentration of each gas is obtained from a known absorption- concentration calibration curve stored in a computer.

An intrinsic distributed optical-fiber gas sensor for detecting the leakage of cryogenically stored gases such as CH4, C2H4, and N2 has also been developed. The sensor’s operation is based on the temperature-dependent transmission loss of optical fiber—that is, the optical fiber is specially designed so the transmission loss increases with decreasing temperature by choosing the appropriate core and cladding materials. Below the critical temperature, in the region of ?55°C, most of the light has transferred to the cladding layer, and the light in the core is cut off. By connecting this temperature-sensitive fiber between a light source and a detector and monitoring the output light level, the loss of light resulting from a cryogenic liquid in contact with the fiber can be detected directly.

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About the Author

Sabrie Soloman, Ph.D, is the Founder, Chairman and CEO of American SensoRx, Inc.